The present invention relates to electronic circuitry, and more particularly, to voltage converter circuits for electronic circuitry.
Digital electronic circuitry, such as CMOS circuitry used in radio systems, is typically powered by a very stable supply voltage. Typically, a voltage of 1.8V is required. The nominal voltage of some batteries used in electronic devices, such as lithium batteries, can be too high (e.g., 3.3V-4.2V), and may not be sufficiently stable for direct use as a power supply voltage. Furthermore, the voltage output by a battery drops as the battery is discharged. Therefore, a voltage converter can be used that receives an input voltage signal from the battery and generates a stable voltage at a desired voltage level that can be used as a power supply voltage for an electronic circuit. A conventional voltage converter typically includes two stages, namely, a step-down (or buck) converter that transforms the input battery voltage to a fixed lower voltage which is just sufficient to provide the supply voltage to the second stage, and a Low-Drop Out (LDO) regulator circuit that receives the lower voltage from the step-down converter and responsively provides a stable output voltage of, for example, 1.8V.
Two stages may be used instead of one, because a buck converter can make an efficient conversion from high to low voltage, but the voltage output by a buck converter can have too much voltage ripple to directly supply some CMOS circuitry. The LDO regulator can provide a clean output voltage having low ripple. However, the LDO regulator can have poor efficiency when connected directly to a high battery voltage.
The efficiency of the buck converter is typically optimized for large load currents. As the load current is reduced, the efficiency of the buck converter decreases as well. Also, leakage currents in the buck converter can start to play a role at low load currents. This is due to the complex circuitry in the buck converter. Existing buck converter circuits can be operated in a “pulse frequency” mode that can reduce the drop in efficiency at low output currents. However leakage current may still be present, since the complex circuitry that maintains an accurate output voltage is still enabled. The problem of leakage current can reduce the standby time of some electronic devices to less than half of what would be possible if the leakage current were not present.